1. Field of the Invention
The present invention relates to a color separating-combining optical system configured to separate illumination light into a plurality of color spectrums, guiding these color spectrums to respective image display elements, and combining the color spectrums modulated by the image display elements, and an image display optical system and projection image display apparatus using it.
2. Description of Related Art
The projection image display apparatus including a combination of reflective liquid crystal display elements with polarizing beam splitters is disclosed, for example, in U.S. Pat. No. 6,183,091. The projection image display apparatus described in this US patent incorporates first, second, third, and fourth polarizing beam splitters 118, 120, 128, 124 and three color-selective phase differentiate plates (retarder stacks) 116, 126, 134, as shown in FIG. 8.
A color-selective phase differentiate plate is a retarder stack that acts so as to rotate a polarized direction of light in a predetermined wavelength region among the visible wavelength region by 90° but keep a polarized direction of light of the other wavelengths unchanged.
In the apparatus shown in FIG. 8, illumination light exiting a light source 100 is aligned into linearly polarized light (S polarized light) by a polarization changer 114, the first color-selective phase differentiate plate 116 rotates a polarized direction of only light of blue (B) out of the linearly polarized light (S polarized light) by 90° (into P polarized light), and the first polarizing beam splitter 118 receives the light to transmit the B light of P polarized light and reflect the light of green (G) and red (R) of S polarized light excluding the B light, thereby effecting color separation. The light of B (P polarized light) travels through the second polarizing beam splitter 120 and impinges on a reflective liquid crystal display B 122. The light of G and R enters the second color-selective phase differentiate plate 126, the phase differentiate plate 126 rotates only the polarized direction of G by 90° (into P polarized light), and the second polarizing beam splitter 120 transmits the G light of P polarized light and reflects the R light of S polarized light, thereby effecting color separation.
The separate light spectrums of G and R impinge on a reflective liquid crystal display G 132 and on a reflective liquid crystal display R 130, respectively.
The P polarization component out of the B light modulated by the reflective liquid crystal display B 122 travels straight through the second polarizing beam splitter 120 and returns toward the light source 100, while the S polarization component thereof is reflected in the second polarizing beam splitter 120 to become projected light. The S polarization component of the R light modulated by the reflective liquid crystal display R 130 is reflected in the third polarizing beam splitter 128 and returns toward the light source 100, while the P polarization component thereof travels straight through the third polarizing beam splitter 128 to become projected light. Furthermore, the P polarization component of the G light modulated by the reflective liquid crystal display G 132 travels straight through the third polarizing beam splitter 128 and returns toward the light source 100, while the S polarization component thereof is reflected in the third polarizing beam splitter 128 to become projected light.
The projected light of G and R is incident into the third color-selective phase differentiate plate 134, by which only the polarized direction of G is rotated by 90°. Therefore, the light spectrums of G and R are aligned both into P polarized light. Then the light of G and R travels through the fourth polarizing beam splitter 124 and the B light of S polarized light is reflected in the fourth polarizing beam splitter 124. This results in combining the light spectrums of R, G, and B into one and projecting the composite light as a color image onto a projection surface.
In the projection image display apparatus proposed in Japanese Patent Application Laid-Open No. 2001-154152, a sheet polarizer (polarizing plate) is placed between an illumination optical system and a polarizing beam splitter as color separating-combining means or between a polarizing beam splitter and a projection lens so as to obtain an image with high contrast.
In general, the polarizing beam splitters demonstrate the ideal polarization separation performance as shown in FIG. 9, for light incident at 45°, but have the imperfect characteristics as shown in FIG. 10, for light incident at angles except for 45°, however.
This occurs for the following reason: in an optical thin film formed in the polarizing beam splitters, where n represents the refractive index of the thin film, d the thickness of the thin film, and θ the angle of incidence of light, the optical thin film acts according to nd cos θ of its optical performance, so that the optical performance varies depending upon the angle θ of incidence.
In the projection image display apparatus as described above, because the light illuminating the reflective liquid crystal displays is light beams having some angular spread 2ω (2ω is determined by the illumination system), the light with the spread of 45°±ω is incident into the polarizing beam splitters. For this reason, the P polarization component and the S polarization component are not perfectly separated in the polarizing beam splitters, so that the light incident to the liquid crystal displays is not perfect linearly polarized light. This results in decrease of contrast and thus poses a problem of degradation of quality of the projected image.
Furthermore, the polarization changer used in the illumination system is an element that aligns the lamp light including the mixture of P polarized light and S polarized light, into a predetermined polarization orientation, but the efficiency of the conversion is not 100%. Therefore, there remains an undesired polarization component at a considerable rate.
In the ordinary polarizing beam splitters, a balance is achieved, for example, between a ratio of reflection of P polarized light and a ratio of transmission of S polarized light, and for this reason, for example, light reflected by a polarizing beam splitter configured to reflect the P polarized light includes a measure of the S polarization component.
While the reflective liquid crystal display is displaying black, the P polarized light incident into the polarizing beam splitter is reflected in the P polarization orientation in the polarizing beam splitter and returns toward the light source, but because of the characteristics of the polarizing beam splitter, part of the S polarized light mixed in the illumination light impinging on the reflective liquid crystal display, travels straight through the polarizing beam splitter and toward the projection lens.
If polarizers are placed on the entrance side and on the exit side of the polarizing beam splitter by applying the configuration proposed in Japanese Patent Application Laid-Open No. 2001-154152, the aforementioned problem of contrast decrease caused by the characteristics of the polarizing beam splitter can be solved, but another problem of decrease in brightness will arise, because the transmittance of the polarizers is not 100%.
In FIG. 8, most of the S polarized light for display of the image of B is reflected by the polarization separating surface of the second polarizing beam splitter 120 and travels through the fourth polarizing beam splitter 124 toward the projection surface, but part thereof travels toward the first polarizing beam splitter 118. Furthermore, part of the light traveling toward the first polarizing beam splitter 118 is reflected by the polarization separating surface of the first polarizing beam splitter 118 and then travels toward a surface 118a, which is neither an entrance surface nor an exit surface in the first polarizing beam splitter 118.
Then the light incident to the surface 118a is reflected there and travels through the first polarizing beam splitter 118 and into the third polarizing beam splitter 128, because of influence of so-called phase jump upon reflection or deviation from the ideal action of the polarizing beam splitter for the light with inclination as described previously. The light entering the third polarizing beam splitter 128 is then incident to the R and G liquid crystal panels 130, 132 and part thereof is finally projected onto the projection surface.
When the so-called leaking light at the polarization separating surface passes through the primarily unexpected paths as described, it can pose the problem of decrease in the contrast of the projected image. When the light modulated by the different color liquid crystal panels is projected onto the projection surface, it can pose a problem of variation in the color tone.
Furthermore, because the light in the color separating-combining optical system is converging light, the light turning into stray light may arrive even at surfaces parallel to the plane where principal rays pass. The light is also reflected on such surfaces to pose a problem that the interior of the color separating-combining optical system is filled with the stray light.